Vehicles have been developed to perform an engine stop when specific engine idle-stop conditions are met and then to automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduced exhaust emissions, reduced vehicle noise, and the like. In some idle-stop systems, engine speed is limited during an engine restart by loading the engine via an alternator mechanically coupled to the engine. However, during engine restarting a substantial amount of current is required by a starter from a battery to start the engine. Consequently, when a higher amount of current is drawn from the battery, the battery voltage may be lowered and the mechanical load provided by the alternator to the engine may change in an unexpected and/or unpredictable manner.
Systems that attempt to limit engine speed by simply applying a substantially constant battery voltage to the alternator field coil during an engine start when the voltage of the battery is used by the starter to crank the engine have limited capabilities to control engine speed during engine starting. In particular, when a substantially constant voltage is applied to an alternator field coil, the current flow in the alternator field coil can be related to the coil impedance and the velocity of the alternator rotor rather than being controlled to a specific value. And, since the load provided by the alternator to the engine is related to the magnetic field strength of the alternator, applying a substantially constant battery voltage to an alternator field coil may not yield a desired level of alternator load control.
The inventors herein have developed a system for controlling engine speed during an engine start, comprising: an engine; a first battery in electrical communication with an engine starter during an engine start; an alternator mechanically coupled to the engine, the alternator having a field coil that is electrically buffered from the battery during an engine start, the field coil in electrical communication with a power source other than the battery during the engine start; and a circuit to vary a strength of a magnetic field produced by the field coil via adjusting field coil current.
By adjusting alternator field current supplied via a power source that is electrically buffered from the battery used to supply power to crank an engine during engine starting, it may be possible to provide different loads to the engine during different engine starting conditions so that engine starting can be improved. For example, alternator field current can be reduced at lower engine temperatures so that higher engine friction at lower engine temperatures may be compensated. Further, alternator field current can be reduced when the engine is started at higher altitudes where engine starting torque may be reduced since less air may be available to start the engine. And, since the alternator field current is supplied from a power source that is electrically buffered from the battery used to crank the engine, alternator field current control can be simplified and may be more robust.
The present description may provide several advantages. For example, the approach can offer flexible alternator field coil current control during a variety of engine operating conditions so that engine starting speed may be more consistent. Further, the approach may simplify alternator field coil current control since the field current is buffered from the battery used to start the engine. By electrically buffering the field coil power source from the battery used to start the engine, it may be possible to mitigate the effect battery voltage droop may have on alternator field control. Further still, the approach may allow alternator field strength to be adjusted according to a number of cylinder combustion events since engine start so that the alternator field can be controlled in response to engine operation rather than time. As such, the alternator field may be controlled such that alternator load can be adjusted in a more repeatable manner.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.